1. Field of the Invention
The present invention relates to a solid-state image sensing apparatus, and more particularly, to a solid-state image sensing apparatus provided with a solid-state image sensing device having a characteristic that an output voltage thereof natural-logarithmically varies in accordance with a light reception amount.
2. Description of the Prior Art
A conventionally-used solid-state image sensing device has a characteristic that an output voltage thereof linearly varies as a light reception amount varies. In the solid-state image sensing device having such a characteristic, each pixel has a different sensitivity. Due to this difference in sensitivity, an output value of each pixel is different from one another even if uniform light is irradiated.
FIG. 1 graphically shows a relationship between a light reception amount L and an output voltage VO of the above-mentioned conventional solid-state image sensing device. The relationship is expressed as: EQU V0.varies.L
When there is a difference among a sensitivity ai of each pixel, an output voltage Vi thereof is expressed as: EQU Vi.sup..varies. ai.times.Li
where Li is a light reception amount of each pixel and i=1, 2, 3, . . . when uniform light L is irradiated onto the solid-state image sensing device, that is, L=L1=L2=L3= . . . , the output voltage Vi of each pixel is also expressed as: EQU Vi.sup..varies. ai.times.L
Thereby, it is understood that there is a difference among an output voltage of each pixel. In order that the difference among an output voltage of each pixel is zero, the output voltages Vi of all the pixels have to be equal to one another when the uniform light L is irradiated. If the sensitivities ai of all the pixels are equal, the output voltages Vi of all the pixels are equal. It is virtually impossible, however, to equalize the sensitivities ai of all the pixels.
In the conventional solid-state image sensing apparatus, to solve the problem, the difference among an output of each pixel is corrected by the subsequently-described calculation. That is, the difference is corrected by multiplying the output voltage Vi of a corresponding pixel by an inverse number of the sensitivity ai of each pixel. An output voltage Vi' obtained through the correction is expressed as: ##EQU1## By the above calculation, the relationship between a light reception amount and an output voltage which relationship differs depending on each pixel can be caused to coincide with a reference line as shown in FIG. 2. Consequently, there is no problem in regarding that each pixel has an equal light reception amount/output voltage characteristic.
However, concerning a solid-state image sensing apparatus provided with a solid-state image sensing device having a characteristic that an output voltage thereof natural-logarithmically varies as a light reception amount varies, since it has a photoelectric conversion characteristic different from that of the above-described conventional solid-state image sensing device, it is impossible to correct the inequality in sensitivity of each pixel by the above-described method.
In a conventional solid-state color image sensing apparatus, since a photoelectric conversion characteristic thereof is linear, subtraction and multiplication/division are required in order to obtain white balance. That is, in the conventional solid-state color image sensing apparatus (where R (red), G (green) and B (blue) filters are provided), the following expressions hold: EQU V.sub.Ri (L)=.intg.a.sub.Ri L.sub.R dt+d.sub.Ri =a.sub.Ri .intg.{.intg..alpha..sub.Ri (.lambda.)L(.lambda.)d.lambda.}dt+d.sub.Ri( 1) EQU V.sub.Gj (L)=.intg.a.sub.Gj L.sub.G dt+d.sub.Gj =a.sub.Gj .intg.{.intg..alpha..sub.Gj (.lambda.)L(.lambda.)d.lambda.}dt+d.sub.Gj( 2) EQU V.sub.Bk (L)=.intg.a.sub.Bk L.sub.B dt+d.sub.Bk =a.sub.Bk .intg.{.intg..alpha..sub.Bk (.lambda.)L(.lambda.)d.lambda.}dt+d.sub.Bk( 3)
where:
V.sub.Ri (L) is an output of an ith pixel of an R channel at an illuminance L; PA1 V.sub.Gj (L) is an output of a jth pixel of a G channel at an illuminance L; PA1 V.sub.Bk (L) is an output of a kth pixel of a B channel at an illuminance L; PA1 a.sub.Ri is a gain of an ith pixel of the R channel; PA1 a.sub.Gj is a gain of a jth pixel of the G channel; PA1 a.sub.Bk is a gain of a kth pixel of the B channel; PA1 d.sub.Ri is a dark output of an ith pixel of the R channel; PA1 d.sub.Gj is a dark output of a jth pixel of the G channel; PA1 d.sub.Bk is a dark output of a kth pixel of the B channel; PA1 L.sub.R is an illuminance of an R component of light; PA1 L.sub.G is an illuminance of a G component of light; PA1 L.sub.B is an illuminance of a B component of light; PA1 .alpha..sub.Ri (.lambda.) is a spectral transmission factor of an ith pixel of the R channel; PA1 .alpha..sub.Gj (.lambda.) is a spectral transmission factor of a jth pixel of the G channel; PA1 .alpha..sub.Bk (.lambda.) is a spectral transmission factor of a kth pixel of the B channel; and PA1 L (.lambda.) is a spectral characteristic of a subject light.
At this time, a gain a, a dark output d and a spectral transmission factor .alpha. take different values depending on each pixel. In the conventional solid-state color image sensing apparatus, first, a dark output is subtracted from an output of each pixel in order to correct a difference among the dark output of each pixel. When a result of the subtraction is V', EQU V'.sub.Ri (L)=a.sub.Ri .intg.L.sub.R dt=a.sub.Ri .intg.{.intg..alpha..sub.Ri (.lambda.)L(.lambda.)d.lambda.}dt(4) EQU V'.sub.Gj (L)=a.sub.Gj .intg.L.sub.G dt=a.sub.Gj .intg.{.intg..alpha..sub.Gj (.lambda.)L(.lambda.)d.lambda.}dt(5) EQU V'.sub.Bk (L)=a.sub.Bk .intg.L.sub.B dt=a.sub.Bk .intg.{.intg..alpha..sub.Bk (.lambda.)L(.lambda.)d.lambda.}dt(6)
Thereby, the difference among the dark output of each pixel is corrected. However, since a difference among the gain a of each pixel and a difference among the spectral transmission factor .alpha. of each pixel still remain, an output V' (W) obtained when a white light W is incident differs.
That is, EQU V'.sub.Ri (W).noteq.V'.sub.Gj (W).noteq.V'.sub.Bk (W).
To correct this, the following calculation is performed: EQU V'.sub.Ri (W)=a.sub.Ri .intg.L.sub.WR .times.dt=a.sub.Ri .intg.{.intg..alpha..sub.Ri (.lambda.)L.sub.w (.lambda.)d.lambda.}dt(7) EQU V'.sub.Gj (W)=a.sub.Gj .intg.L.sub.WG .times.dt=a.sub.Gj .intg.{.intg..alpha..sub.Gj (.lambda.)L.sub.w (.lambda.)d.lambda.}dt(8) EQU V'.sub.Bk (W)=a.sub.Bk .intg.L.sub.WB .times.dt=a.sub.Bk .intg.{.intg..alpha..sub.Bk (.lambda.)L.sub.w (.lambda.)d.lambda.}dt(9)
where L.sub.w is an illuminance of each color component of the white light W.
In order to correct so that all the outputs obtained at that time equal VO, each output V' (W) is multiplied by .beta. with respect to which the following expressions hold: ##EQU2## Therefore, all the outputs obtained when white light is irradiated are equalized by ##EQU3## This is because EQU V".sub.Ri (W)={V0/V'.sub.Ri (W)}.times.V'.sub.Ri (W)=V0 EQU V".sub.Gj (W)={V0/V'.sub.Gj (W)}.times.V'.sub.Gj (W)=V0 EQU V".sub.Bk (W)={V0/V'.sub.Bk (W)}.times.V'.sub.Bk (W)=V0.
As described above, white balance is obtained by performing multiplication and division after a dark output is subtracted.
However, the above-described method where white balance is obtained by multiplication and division is disadvantageous in that a complicated arrangement is required for the circuit and that the processing takes time in a case where the multiplication and division are performed in a software manner. Further, when a color temperature of a light source changes, according to the conventional method, it is necessary to obtain white balance again by performing the multiplication and division again, since the previously-mentioned LW(.lambda.) changes. That is, it is necessary to perform the multiplication and division every time the color temperature changes. As a result, a more complicated arrangement is required for the circuit in a case where correction is made with respect to the circuit, and more time is required because of the complicated calculation in a case where correction is made with respect to the software.